1. Field of Invention
This invention relates to the removal and recovery of liquid pollutants which have contaminated the groundwater aquifer, and in particular, to those pollutants which are substantially immiscible with water and rest upon and travel with the groundwater.
2. Description of Related Art
During the past decade, one of the major environmental concerns has been the detection and removal of pollutants which threaten the underground water supply of the United States. Of particular concern has been the pollution by organic compounds such as chlorinated hydrocarbons and petroleum derivatives. Hydrocarbon pollution, in particular, has been acute where gasoline storage tanks and transmission lines have leaked or ruptured at refining facilities, distribution sites, or along the length of the transmission lines. As the leaking hydrocarbons disperse into the ground, a plume is formed, narrow at the top and spreading out until it contacts the water table. The actual shape of the plume will depend on the porosity of the soil in various directions as well as the horizontal flow of the water table. Once the hydrocarbon has reached the water table, it is free to move with the underground water.
Removal techniques have utilized the fact that the pollutants frequently have a lower density than water and therefore ride on the top of the water table. Two approaches have been taken to recovering the pollutants from the groundwater both utilizing wells with perforated casings which are constructed so as to intercept the groundwater. In the first approach, passive accumulators are placed in the well at the pollutant/water interface. These devices collect a volume of pollutant and then discharge it above ground. Examples are the collection chamber of Swink, U.S. Pat. No. 3,915,225; the automatic skimmer of Moyer, U.S. Pat. No. 4,404,093; and the semipermeable membrane collector of Breslin, U.S. Pat. No. 4,497,370. These devices are passive in the sense that they intercept only the pollutants which flow through the well as a consequence of normal underground water movement.
In the second approach, an attempt is made to intercept a higher proportion of the pollutant by drawing the pollutant to the well site using the groundwater itself. In this technique, water is pumped from the well at a rate sufficient to depress the water level in the well below its natural level. As a consequence of this water level depression, water from the surrounding area, under the influence of gravitational hydrostatic pressure, flows towards the well trying to reestablish the original level. As the water flows towards the well, it carries upon it the lower density pollutant. A so called `cone of depression` is formed with the well at its apex. The theory behind and the hydraulics of such a cone of depression have been described in detail by Fletcher G. Driscoll in Groundwater and Wells, (Second Edition 1986, Johnson Division, St. Paul, Minn.) In cross section, both the level of the water table and the pollutants riding upon it are lowest in the well. In this manner, the well intercepts a larger volume of pollutant. Examples of this approach are Solomon, U.S. Pat. No. 4,273,650; Farmer, U.S. Pat. No. 4,469,170; McLaughlin et al., U.S. Pat. Nos. 4,527,633, 4,546,830, and 4,625,801; and Harlow, U.S. Pat. No. 4,625,807.
Even in the prior art approaches using the cone of depression technique, the pollutant is first accumulated in a fixed volume reservoir, which is then purged and allowed to refill. The overall rate of pollutant removal is, therefore, dependant on the reservoir size as well as the reservoir discharge and refill times. In the present system this limitation is overcome by continuous pumping and removal of the pollutant as long as the pollutant intake remains in the pollutant layer.
In Solomon and Farmer, the pollutant is removed by an electrically driven pump, while in McLaughlin et al. and Harlow the pollutant is removed by compressed gas. In these cases, the presence and filled level of the pollutant in the reservoir is registered by various appropriate sensors. Solomon and Swink placed both the reservoir and electric pump at the pollutant/water interface. McLaughlin et al. and Harlow believed that having an electrically driven pump in proximity to potentially explosive pollutants at the pollutant/water interface was an unecessary risk and, therefore, designed their systems to use compressed air to remove the pollutant. In the present system the advantage of an electrically driven pollutant pump is retained while the pump is located away from the water/pollutant interface submerged below the surface of the water.
In both the passive and cone of depression approach, one of the major problems is positioning the pollutant receiving unit so that it remains in the pollutant/water interface. The interface position in both approaches is constantly changing since the water table is not static. Its position will depend on the porosity of the soil, groundwater flow rates, and the groundwater recharge rate which is principally a function of rainfall. Frequent observation and readjustment of the reservoir units is, therefore, necessary. To help avoid this problem, Farmer designed a floating skimmer reservoir which moves with the interface. However, a penalty is paid in Farmer's approach in the requirement for a larger diameter well and multiple suspension systems for the water pump, skimmer, and pollutant pump.
In addition, significant changes in the depth of the water table due to rising and falling water levels disturb the flow of pollutant into the well by forcing the pollutant back and forth into the soil matrix. For this reason, the best way to effect pollutant removal is to establish and hold a cone of depression within as short a vertical range as possible wherein a flow of pollutant out of the soil matrix is established and maintained. The present invention provides a sensor and control system which minimizes as much as possible variations in water table depth. When the hydraulics of a given well system exceed the ability of the water pump to hold the depressed level at a given depth, the design of the present invention permits a rapid and easy repositioning of the sensing probe and, thereby, the cone of depression so that the control system responds to the new conditions.
Further, all of the disclosed cone of depression technique devices require at least two separate suspension systems, one for the water pump and one for the pollutant recovery reservoir. McLaughlin et al. suggest but do not show a single suspension system. In addition to the multiple suspension systems, either multiple air hoses or electric lines, all of which are subject to abrasion and stretch, are required by the prior art systems. The present system avoids the problems of flexible suspension systems by using a rigid riser tube to suspend the pumps. The riser tube also protects the power cable from unnecessary abrasion as well as satisfying the National Electrical Code requirements.
Because of the physical size and number of units and suspension systems which must be employed, many of the prior art systems require wells which can range up to 24 inches in diameter. Wells of this size are time consuming and expensive to construct. On the other hand, wells of approximately 4 inches in diameter are relatively easy and inexpensive to construct. Typically, in the past, smaller wells of this size were constructed surrounding the larger recovery well for observation purposes. The present system and its components are designed to utilize the smaller diameter wells.